July 16 (Bloomberg) -- Britain will expel four Russian diplomats because of their country's refusal to extradite former KGB agent Andrei Lugovoi so he can stand trial for the murder of Alexander Litvinenko.

``A U.K. citizen has suffered a horrifying and lingering death,'' Foreign Secretary David Miliband told Parliament in London today. ``Given the seriousness of the crime, Russia's reply to the extradition request is extremely disappointing.''

Litvinenko, 43, died in London in November after being poisoned by radioactive polonium-210. The U.K. Crown Prosecution Service accused Lugovoi of the murder and sought his extradition. Both Lugovoi and the Kremlin deny involvement. Russia turned down the request, saying its constitution bars extradition.

The case has strained relations between Russia and the U.K., already troubled by Britain's refusal to extradite exiled billionaire Boris Berezovsky, who is wanted in Russia on a range of charges including fraud. Today's expulsions mark the biggest rift between the two countries since 1996, when first Russia, then Britain expelled four diplomats over a spying dispute.

This is the latest indication of the ``serious deterioration'' in relations with Russia, said Michael Moore, a member of Parliament from Britain's Liberal Democrat Party. He called on the European Union to remain united across the range of issues where they were seeking to change Russian policy.

Rising Tensions

The issue adds to more general tensions between Russia and the West that have grown as the U.S. pushes a plan to build a missile defense system in the Czech Republic and Poland. The European Union also is seeking independence for Kosovo from Serbia, a Russian ally. Two days ago, Russian President Vladimir Putin suspended an arms control agreement with the North Atlantic Treaty Organization.

The head of the Russian lower house of parliament's foreign affairs committee, Konstantin Kosachyov, said the U.K. was being arrogant in demanding that Russia override its constitution and extradite Lugovoi.

``You can act this way toward a banana republic, but Russia is not a banana republic,'' Kosachyov said in comments broadcast on state channel Rossiya.

Russian President Vladimir Putin's spokesman had no immediate comment. Miliband said he spoke with Russian Foreign Minister Sergei Lavrov earlier today.

Russia's Prosecutor General said last week that it has sought the extradition of 21 people from Britain since 2002. Of these, the U.K. has refused to extradite 11, given asylum to six and kept the other four cases under review.

Visa Clampdown

Miliband also suspended ``visa facilitation negotiations'' aimed at smoothing the process Russians take to get permission to visit the U.K. He also said some practices in granting visas to Russians had changed, though he didn't detail those measures.

Lugovoi met Litvinenko at the Millennium Hotel in London's Mayfair on Nov. 1, the day he became ill. Lugovoi has repeatedly denied any involvement in the killing and has accused British secret services of being behind the death.

Litvinenko, who became a British citizen, had been granted political asylum in the U.K. and was a friend of Berezovsky.

After the poisoning of Litvinenko, polonium traces were found in at least 12 locations across London, including an Itsu sushi restaurant on Piccadilly. At least 137 people tested positive for the substance, including the ex-spy's widow, Marina Litvinenko. In most cases, the doses were too small to be detrimental, though a longer-term threat to health was posed to 17 people, according to the U.K.'s Health Protection Agency.

Miliband's Discussions

Miliband, who received support from opposition lawmakers, sought to emphasize the importance of Britain's economic relationship with Russia, saying he spoke with officials at the Confederation of British Industry today and that he will meet with executives who do business in Russia tomorrow.

``We have thought very, very carefully about this,'' Miliband said. ``The response I have announced today is proportionate. This is not an anti-Russian statement. The clear desire is to have a close relationship with the Russian government.''

Britain's opposition Conservative Party backed Miliband's response to the incident.

``This issue must not simply be allowed to fade,'' said William Hague, a Conservative lawmaker who speaks on foreign affairs. ``Russia must abide by international rules if it is to receive the assistance and cooperation it seeks on other issues.''

To contact the reporters on this story: Mark Deen in London at markdeen@bloomberg.net

Last Updated: July 16, 2007 12:02 EDT